The invention relates to a continuous process for the autothermal gas-phase dehydrogenation of a hydrocarbon-comprising gas stream and regeneration of the catalyst which is configured as a monolith, and a use of the process.
Ceramic or metallic monoliths have become established as catalyst supports for noble metal catalysts in mobile and stationary offgas purification. The channels offer a low flow resistance to the gas stream while at the same time allowing uniform accessibility to the outer catalyst surface for gaseous reaction media. This is advantageous compared to irregular beds in which numerous diversions in the flow around the particles result in a large pressure drop and the catalyst surface may not be uniformly utilized. The use of monoliths is of general interest for catalytic processes which have high volume flows and in which the reaction is carried out adiabatically at high temperatures. In chemical production engineering, these features apply particularly to dehydrogenation reactions which proceed in a temperature range from 400° C. to 700° C.
Advances in catalyst technology make it possible to carry out the selective combustion of the dehydrogenation hydrogen in the presence of hydrocarbons, as described, for example, in U.S. Pat. No. 7,034,195. Such a mode of operation is referred to as autothermal dehydrogenation and allows dehydrogenation reactors to be heated directly, so that complicated apparatuses for indirect preheating and intermediate heating of the reaction mixture are dispensed with. Such a process is, for example, described in US 2008/0119673. However, this process has the serious disadvantage that the dehydrogenation is carried out over a heterogeneous catalyst in pellet form: the high flow resistance of pellet beds requires a large reactor cross section and a correspondingly low flow velocity in order to limit the pressure drop in the catalytically active bed. This disadvantage is compensated by means of a very complicated apparatus for introducing and distributing the oxygen, which partly negates the advantage of autothermal dehydrogenation.
The European patent application EP 09 177 649.2, which is not a prior publication, discloses a reactor and a process for the autothermal gas-phase dehydrogenation of hydrocarbons using heterogeneous catalysts configured as monoliths, which ensures control of the combustible reaction media at the high reaction temperatures, frequently in the range from about 400 to 700° C., and allows simple accessibility and handling of the monoliths, in particular when equipping the reactor or in the case of catalyst replacement.
EP 09 177 649.2 provides a reactor in the form of an essentially horizontal cylinder for carrying out an autothermal gas-phase dehydrogenation of a hydrocarbon-comprising gas stream by means of an oxygen-comprising gas stream over a heterogeneous catalyst configured as a monolith to give a reaction gas mixture, where                the interior space of the reactor is divided by means of a detachable cylindrical or prismatic housing G which is arranged in the longitudinal direction of the reactor and is gastight in the circumferential direction and is open at both end faces into        an inner region A which has one or more catalytically active zones and in which a packing composed of monoliths stacked on top of one another, next to one another and behind one another is provided in each catalytically active zone and a mixing zone having fixed internals is provided before each catalytically active zone and        an outer region B arranged coaxially with the inner region A,        with one or more feed lines for the hydrocarbon-comprising gas stream to be dehydrogenated into the outer region B, diversion of the hydrocarbon stream to be dehydrogenated at one end of the reactor and introduction via a flow equalizer into the inner region A,        with one or more, independently regulable feed lines, with each feed line supplying one or more distributor chambers, for the oxygen-comprising gas stream into each of the mixing zones and        with a discharge line for the reaction mixture of the autothermal gas-phase dehydrogenation at the same end of the reactor as the feed line for the hydrocarbon stream to be dehydrogenated.        
At the end of the reactor at which the discharge line for the reaction gas mixture of the autothermal gas-phase dehydrogenation is arranged, it is advantageous to provide a shell-and-tube heat exchanger having a bundle of tubes through which the reaction gas mixture of the autothermal gas-phase dehydrogenation is passed and intermediate spaces between the tubes through which the hydrocarbon-comprising gas stream to be dehydrogenated is passed in countercurrent to the reaction gas mixture of the autothermal gas-phase dehydrogenation.
However, EP 10 196 216.5 describes an improved reactor for autothermal gas-phase dehydrogenation, which has safety advantages and also solves the problems of sealing the shell-and-tube heat exchanger.
The known reactors for autothermal gas-phase dehydrogenation are operated with two reactors of the same type being provided and a first reactor being operated in the functional mode of the autothermal gas-phase dehydrogenation until the activity of the catalyst decreases to such an extent that it has to be regenerated, whereupon the reactor is switched over to the regeneration mode and a second reactor of the same type is switched to the production mode of the autothermal gas-phase dehydrogenation.
Plants for autothermal gas-phase dehydrogenation generally produce very large product streams, frequently of an order of magnitude of from 150 000 to 200 000 metric tons per annum, which after the dehydrogenation are passed to further process steps, i.e., in particular, work-up and/or reaction steps. These process steps have to operate continuously since in the case of the large mass flows a fresh start or a change in load would be too complicated.
In addition, in the case of the mode of operation according to the prior art using two reactors which are operated alternately in the production mode and the regeneration mode, the outlay in terms of capital costs, safety, working time, etc., for switching over between the two modes of operation is high in industrial plants. A scale-up is complicated because two reactors have to be made appropriately larger to achieve an increase in capacity. Furthermore, a buffer vessel is generally necessary in the mode of operation according to the prior art using two reactors operated alternately in the production mode and the regeneration mode in order to compensate for the switch-over time.